The present invention is directed toward an apparatus for a loud audible signal using a minimum of space and power. More specifically, the present invention is directed toward a system comprising a piezoelectric transducer and an integrated circuit which contains a number of Schmitt triggers.
A variety of products from automobiles to household appliances rely upon effective alarms to notify the user of a wide variety of operational messages, including safety concerns. However, currently available alarms are unacceptable or, at best, inefficient in terms of cost, energy requirements, and complexity. Thus, a simple, inexpensive alarm utilizing a low power source is desired.
Piezoelectric transducers are sound producing electronic devices that are preferred by industry because they are by and large extremely inexpensive, reliable, durable, and versatile. This transducer has the unique property that it undergoes a reversible mechanical deformation on the application of an electrical potential across it. Conversely, it also generates an electrical potential upon mechanical deformation. These characteristics make it highly desirable for sound producing applications. When an oscillating potential is placed across the transducer, it vibrates at roughly the same frequency as the oscillations. These vibrations are transmitted to the ambient medium, such as air, to become sound waves. Piezoelectric transducers can also be coupled to a simple circuit in what is known as a feedback mode, well known in the art, in which there is an additional feedback terminal located on the element. In this mode, the crystal will oscillate at a natural, resonant frequency without the need for an external source for applying continuous driving oscillations. As long as the oscillations are in the range of audible sound, i.e., 20 to 20,000 Hertz, such oscillations can produce an audible signal for use as an alarm or an indicator.
Any periodic oscillation can be characterized by at least one amplitude and frequency. Ordinarily, the amplitude of oscillations of interest in a piezoelectric transducer application will be dictated by the voltage swing applied across the element. By the principles explained above, it is evident that there will be a greater mechanical deformation in the crystal with greater applied voltage. The effect is roughly linear within limits, those limits based in general on crystal composition and geometry. Thus, in the linear region, doubling the voltage swing doubles the mechanical deformation. Doubling the mechanical deformation significantly increases the amplitude of vibrations transmitted into the ambient medium. Increased amplitude of vibrations in the medium causes an increased sound level, the relationship determinable by well known physical equations.
More specifically, when a piezoelectric element possesses two terminals and a driving oscillation is placed across one while the other is clamped to a common potential such as ground, the voltage swing will be at most the amplitude of the oscillations. Thus, if an oscillation of amplitude of 5 volts is placed across one terminal, while the other is maintained at 0 volts, the maximum voltage swing will be 5 volts. This effectively caps the achievable decibel level of any sound to a value corresponding to the supply voltage. One could double the supply voltage to achieve double the voltage swing, but this has the disadvantage of added cost, and further is impractical when a piezoelectric audio circuit is to be placed in a unit having a standardized voltage supply such as an automobile. Alternatively, one could use a second supply disposed to provide the same oscillations but in a reversed polarity to double the effective voltage swing. But this approach possesses at least the same disadvantages.
As shown by the present invention, when a piezoelectric element possesses two terminals and a driving oscillation is placed across one, and the identical driving oscillation is placed across the other but shifted 180 degrees out of phase, the voltage swing will be about two times the amplitude of the oscillations. By "180 degrees out of phase" it is meant that each terminal generates a signal having a substantially square wave form, wherein one wave form is high and the other is low at any given time. Thus, if an oscillation having an amplitude of 5 volts is placed across one terminal while the other experiences the same oscillation but separated by 180 degrees of phase (half the period of the cycle), then the maximum voltage swing will be 10 volts. Higher sound pressures and louder tones result with a voltage swing of 10 volts than with a voltage swing of 5 volts.
As shown by the present invention, the phase shift needed to effectively double the voltage swing across the transducer can be accomplished by use of one or more Schmitt triggers. It is believed that Schmitt triggers are particularly useful to the present invention because of the fast switching time and because they require minimal addition of components. Schmitt triggers are a special type of bistable amplifier circuit known in the art which can sustain two different voltages, each being equal in amplitude but 180 degrees out of phase. Schmitt triggers further have regenerative capability through the use of a feedback loop. In other words, Schmitt triggers can be started or triggered by an initial pulse of only a short duration and can be maintained indefinitely (for all practical purposes) in one of their bistable states through its own feedback, without the need for an external source to supply continuing driving oscillations. Furthermore, Schmitt triggers have the added benefit of producing either a high or low output in response to a trigger signal, depending upon the state that the circuit is already in. In other words, where the input voltage is between the low and high threshold voltages of each of the stable states of a Schmitt trigger, the output of the Schmitt trigger is inverted from high to low, or vice versa. This feature can be used to place alternating voltage drops of equal magnitude across opposing terminals of a transducer, thus increasing the mechanical deformation in the transducer.
Particularly in alarm applications, what is needed is a loud sound that does not depend on the added circuit complexity of a doubled supply voltage or an additional reversed polarity supply. Loud sounds require relatively high voltages to produce relatively large amplitude vibrations in the transducer. In a special analog circuit, this might not be an obstacle. However, in a circuit containing elements that are safely and reliably operable only in a limited range of potentials, accommodations must be made to insure that those elements do not receive an electrical potential that is too high. Thus, in particular when a loud alarm sound is needed, care must be taken to separate the potentials driving the transducer from the potentials driving the more sensitive circuit elements. For example, integrated circuits often have specifications limiting the recommended power supply to 5 volts DC. If one desires to power a transducer using a supply voltage of 16 volts DC, care must be taken to regulate the power supplied to the integrated circuit.
Accordingly, one object of the present invention is inexpensively to enable loud sounds to be generated by an audio circuit that overcomes the foregoing disadvantages.
Still another object of the invention is to enable the use of voltage-sensitive components in the same circuit that contains an audio transducer that is disposed to receive large voltage swings.
Yet another object of the present invention is to provide a simple, inexpensive, low power device that creates a loud alarm for users.
Another object of the present invention is to utilize feedback from the audible alarm to facilitate the continued operation of the alarm.
Yet another object of the present invention is to utilize an array of Schmitt triggers to increase the voltage across a piezoelectric transducer so as to maximize the resonance of the resulting alarm signal.
Still another object of the present invention is to utilize another Schmitt trigger to toggle the oscillation of the circuit on and off, thus creating a distinct, intermittent audible alarm.